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Pregnant Exercisers Test Limits

JAMES PIVARNIK, the director of the Human Energy Research Laboratory at Michigan State University, is all in favor of a woman exercising when she is pregnant. But some of the athletes he studied seemed to know no bounds.

“I had a woman, she was huge, and she was running seven-minute miles up to Week 39,” Dr. Pivarnik said. “It looked like she had a basketball under her shirt.”

He was worried, but not about the baby. Dr. Pivarnik was concerned about the woman’s abdominal muscles and whether they would be pulled with all that jostling. “But she claimed she was not uncomfortable, so who was I to argue?” he said.

Both the baby and the woman’s abdominal muscles turned out to be fine. Dr. Pivarnik’s nonchalance about the effects of strenuous exercise on a fetus might take some women and their doctors aback.

But his was an educated response, he said. He based it on his research and on what he and his colleagues found by surveying published studies and case histories of pregnant athletes who continued to train.

“We looked at training patterns during pregnancy and postpartum,” Dr. Pivarnik said. “And we asked, ‘Was the amount of training related at all to adverse events?’ The answer was no.”

They found athletes who, like the seven-minute miler, surprised even exercise physiologists with their feats. In 2001, for example, Regan Schreiber, an all-American swimmer at Penn State in the 1990s, swam the English Channel (30 miles) in 9 hours 30 minutes. She was 11 weeks pregnant.

(A good swimmer can do three miles an hour, Dr. Pivarnik added.)

The researchers tell of pregnant competitive athletes who significantly elevated their body temperature with strenuous exercise. Doctors often worry that heating a pregnant woman’s body may harm a fetus. But those women had normal babies. And a few small studies of pregnant women who deliberately exercised to raise their body temperature also found no effects on the babies.

The researchers add that questions about exercise and pregnancy remain, making it difficult for doctors to give advice. The problem is deciding what lines to draw, and why. If the risks of exercise were low, but real, case reports and small studies would not find them.

The best way to learn how much exercise is safe is with studies in which pregnant women are randomly assigned to exercise regimens. Such studies, though, “can’t be done,” said Dr. Mona Shangold, the director of the Center for Women’s Health and Sports Gynecology in Philadelphia. No researchers or women would want to take a chance that the exercise might injure the fetus.

As a result, said Dr. Shangold, an expert recommended by the American College of Obstetricians and Gynecologists, doctors tend to play it safe.

But the advice often varies by doctor and can be based more on hunches than science, she said. Dr. Shangold says she tells women to limit their exercise time to 30 minutes a session, which she said was arbitrary advice.

“The problem is that we don’t know yet what the safe limit is,” Dr. Shangold said. “We are probably more conservative than we need to be.”

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Credit
Filip Kwiatkowski for The New York Times

In fact, as Dr. Pivarnik found, advice to pregnant women is all over the map. He and his colleagues asked athletes who were or recently had been pregnant what they had been told.

Some received specific advice, like keep your heart rate below 140 beats a minute. That pretty much guarantees you won’t be exerting yourself much. It was in 1985 guidelines set by the American College of Obstetricians and Gynecologists, but it makes little sense, Dr. Shangold said.

“Heart rate is not a useful parameter to monitor during pregnancy,” she said. “It varies widely during pregnancy and the heart rate response to exercise also varies widely.”

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Other women reported vague advice like “listen to your body.” Others got very conservative advice, like stopping all but the mildest exercise until six weeks after the baby is born.

Emily Mellow, a triathlete who lives in Brooklyn, was told she could continue with her vigorous program during her pregnancy. But if she felt three Braxton-Hicks contractions 10 minutes apart while she was exercising, she should stop. Those sporadic uterine contractions, which are common during pregnancy, could be a danger sign if they happened while she was exerting herself, her doctor said.

During a run, when Ms. Mellow was six months pregnant, she had three Braxton-Hicks contractions. She said she thinks her doctor meant that she should stop that particular exercise session, but, she said, “I was so freaked out because I was six months pregnant that I discontinued running altogether.” She continued to swim but not at her pre-pregnancy level of exertion. Her goal, she said, was “to make my workouts interesting rather than challenging.”

Dr. Shangold said she had never heard of the Braxton-Hicks advice. “That probably is erring on the side of being overly cautious,” she said.

Even women doctors are not always sure what to do.

Dr. Linda Fan, an obstetrician-gynecologist, was taking kickboxing classes and running five miles several times a week when she found out she was pregnant. Dr. Fan, who helps train residents at Columbia University, immediately stopped kickboxing, worried that her heart rate would go too high.

Her obstetrician, a marathon runner, told her she could continue to run.

“She knew being active would make me feel better,” Dr. Fan said. But she took it easy running, worried that if she exercised too hard, oxygen might get shunted to her muscles instead of to her fetus.

At 20 weeks of pregnancy, Dr. Fan switched to walking on a treadmill carrying light weights or using an elliptical cross trainer at her gym while watching television.

DR. VONDA WRIGHT, an orthopedist at the University of Pittsburgh, stopped running when she became pregnant. “I didn’t want to do anything jarring enough to hurt the baby,” she said.

At first, Dr. Wright used an elliptical cross trainer, but when she learned the placenta was low in her uterus, she lowered her exercise intensity, concerned that the placenta might break away from her uterus. She walked on a track and climbed stadium stairs between laps. Toward the end of her pregnancy, she stopped exercising.

Dr. Wright had her baby a few weeks ago and is eager to start running again. Dr. Fan, whose baby is 6 ½ months old, said it was not easy, but she resumed a running program.

She began with a one-mile walk when her baby was two weeks old. Then, she jogged slowly for a couple weeks. “I kind of built up,” she said. “By five weeks, I did my first five-mile run.”

Two months after her baby was born, Dr. Fan said, “I was feeling pretty good.”